Contact lenses may be classified in two general categories, soft and hard lenses. Soft contact lenses are made of a material with a relatively low modulus, such that the lenses are flexible and bendable. Soft hydrogel contact lenses are an example. Hard contact lenses have a much higher modulus and are relatively stiff. One class of hard contact lens materials is RGP copolymers. RGP materials are composed of a silicon-containing copolymer and are able to transmit gases, particularly oxygen. Thus, oxygen can be transmitted through an RGP contact lens and to the cornea while the lens is worn.
U.S. Pat. No. 5,346,976 (Ellis et al.) describes various RGP copolymers. The preferred copolymers of U.S. Pat. No. 5,346,976 have been successful commercially. However, the oxygen permeability of these preferred copolymers is generally no greater than 140 barrers.
Oxygen permeability is a desirable property for contact lens materials since the human cornea will be damaged if it is deprived of oxygen for an extended period. Oxygen permeability is conventionally expressed in units of barrer, also called Dk. Oxygen transmissibility is a property of contact lens materials related to oxygen permeability. Oxygen transmissibility is oxygen permeability divided by lens thickness, or Dk/t. Oxygen permeability of rigid contact lens materials is important. As an example, orthokeratology has gained in popularity in recent years. Orthokeratology involves wearing a rigid contact lens overnight, with the lens being designed to intentionally alter the shape the cornea. As another example, some lens designs, such as toric lenses or multifocal lenses, may be relatively thick, meaning less oxygen is transmitted through the lens. Thus, unless the lens has a sufficiently high oxygen permeability, the lens may not be safe for overnight wear.
Currently, most RGP contact lenses are manufactured by cutting the lens on a lathe. As an example, the RGP copolymer is cast in the form of a rod, the rod is cut into cylindrical disks (also referred to as buttons), and lenses are lathed from these buttons. Thus, an RGP material must have sufficient toughness, and not be brittle, so that it is machineable.
A challenge in developing improved RGP copolymers is that modifying a copolymer to increase oxygen permeability frequently compromises other desired properties of the material, such as machineability or optical clarity. Also, increasing the silicone content to increase oxygen permeability may result in a surface that is not sufficiently wettable by the tear film when worn.